There are many types of signals which may be transmitted (e.g. broadcasted) to a receiver. For example, satellites may be used to transmit signals to satellite receivers. The signals may be any type of transmission signals such as broadcast signals including television signals or radio signals. It is useful for satellites to be in geostationary orbits around the Earth such that they maintain a fixed position relative to points on the Earth's surface. There are many different satellites in orbit around the Earth. A satellite may transmit multiple signals on multiple satellite channels. Frequency division may be used to distinguish between different satellite channels. That is, a satellite channel may be associated with a frequency range in which signals are transmitted on that channel, and there is typically a frequency gap (i.e. a guard interval) between different satellite channels, such that a satellite receiver can more easily distinguish between signals transmitted on different satellite channels.
Satellite broadcast signals might not be well-defined for particular geographical regions, e.g. because the signals from a satellite may be received over a wide area on the Earth's surface. Different satellites may broadcast satellite signals according to different formats. For example, the satellite channels transmitted from a first satellite may differ significantly (e.g. in frequency, bandwidth and/or coding protocol) to the satellite channels transmitted from a second satellite. Furthermore, one satellite may transmit multiple signals from different transponders (i.e. on different channels). A satellite receiver may be able to receive the satellite broadcast signals on multiple channels from a single satellite. Furthermore, by directing a dish of the satellite receiver towards different satellites, the satellite receiver may be able to receive satellite broadcast signals from different satellites. Typically, the frequencies of satellite broadcast signals are constrained to be within a satellite broadcast spectrum. That is, the satellite broadcast spectrum is the range of frequencies in which satellite signals may be broadcast. For example, the satellite broadcast spectrum may be the L-Band of the electromagnetic spectrum, which has frequencies in the approximate range from 1 to 2 GHz.
In order for a satellite receiver to properly receive and decode satellite signals, the satellite receiver first determines the frequencies and symbol rates of the satellite channels which it is receiving. A satellite receiver may do this by performing a two dimensional scan over the entire satellite broadcast spectrum at all frequencies and at all possible symbol rates. Typically, the satellite broadcast spectrum is approximately 1 GHz wide and the symbol rates may correspond to channel bandwidths from approximately 330 kHz to approximately 50 MHz. At each combination of frequency and symbol rate the satellite receiver may test the received signal to see if it can properly decode the signal: if it can properly decode the signal, then it is determined that there is a satellite channel at this frequency and symbol rate; but if it cannot properly decode the signal, then it is determined that there is not a satellite channel at this combination of frequency and symbol rate. A scan of the entire satellite broadcast spectrum performed in this way may take approximately twenty minutes to complete.
As well as satellite broadcast signals, other transmission signals may be transmitted to receivers, such as terrestrial broadcast signals (e.g. for terrestrial television and radio signals) or mobile telephony signals for transmitting signals within a mobile telephone network.